.\" Copyright (C) 2012 Michael Kerrisk <mtk.manpages@gmail.com>
.\" A few fragments remain from a version
.\" Copyright (C) 1996 Free Software Foundation, Inc.
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH init_module 2 2024-05-02 "Linux man-pages 6.9.1"
.SH NAME
init_module, finit_module \- load a kernel module
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.BR "#include <linux/module.h>" "    /* Definition of " MODULE_* " constants */"
.BR "#include <sys/syscall.h>" "     /* Definition of " SYS_* " constants */"
.B #include <unistd.h>
.P
.BI "int syscall(SYS_init_module, void " module_image [. len "], \
unsigned long " len ,
.BI "            const char *" param_values );
.BI "int syscall(SYS_finit_module, int " fd ,
.BI "            const char *" param_values ", int " flags );
.fi
.P
.IR Note :
glibc provides no wrappers for these system calls,
necessitating the use of
.BR syscall (2).
.SH DESCRIPTION
.BR init_module ()
loads an ELF image into kernel space,
performs any necessary symbol relocations,
initializes module parameters to values provided by the caller,
and then runs the module's
.I init
function.
This system call requires privilege.
.P
The
.I module_image
argument points to a buffer containing the binary image
to be loaded;
.I len
specifies the size of that buffer.
The module image should be a valid ELF image, built for the running kernel.
.P
The
.I param_values
argument is a string containing space-delimited specifications of the
values for module parameters (defined inside the module using
.BR module_param ()
and
.BR module_param_array ()).
The kernel parses this string and initializes the specified
parameters.
Each of the parameter specifications has the form:
.P
.RI "        " name [\c
.BI = value\c
.RB [ ,\c
.IR value ...]]
.P
The parameter
.I name
is one of those defined within the module using
.IR module_param ()
(see the Linux kernel source file
.IR include/linux/moduleparam.h ).
The parameter
.I value
is optional in the case of
.I bool
and
.I invbool
parameters.
Values for array parameters are specified as a comma-separated list.
.SS finit_module()
The
.BR finit_module ()
.\" commit 34e1169d996ab148490c01b65b4ee371cf8ffba2
.\" https://lwn.net/Articles/519010/
system call is like
.BR init_module (),
but reads the module to be loaded from the file descriptor
.IR fd .
It is useful when the authenticity of a kernel module
can be determined from its location in the filesystem;
in cases where that is possible,
the overhead of using cryptographically signed modules to
determine the authenticity of a module can be avoided.
The
.I param_values
argument is as for
.BR init_module ().
.P
The
.I flags
argument modifies the operation of
.BR finit_module ().
It is a bit mask value created by ORing
together zero or more of the following flags:
.\" commit 2f3238aebedb243804f58d62d57244edec4149b2
.TP
.B MODULE_INIT_IGNORE_MODVERSIONS
Ignore symbol version hashes.
.TP
.B MODULE_INIT_IGNORE_VERMAGIC
Ignore kernel version magic.
.TP
.BR MODULE_INIT_COMPRESSED_FILE " (since Linux 5.17)"
.\" commit b1ae6dc41eaaa98bb75671e0f3665bfda248c3e7
Use in-kernel module decompression.
.P
There are some safety checks built into a module to ensure that
it matches the kernel against which it is loaded.
.\" http://www.tldp.org/HOWTO/Module-HOWTO/basekerncompat.html
.\" is dated, but informative
These checks are recorded when the module is built and
verified when the module is loaded.
First, the module records a "vermagic" string containing
the kernel version number and prominent features (such as the CPU type).
Second, if the module was built with the
.B CONFIG_MODVERSIONS
configuration option enabled,
a version hash is recorded for each symbol the module uses.
This hash is based on the types of the arguments and return value
for the function named by the symbol.
In this case, the kernel version number within the
"vermagic" string is ignored,
as the symbol version hashes are assumed to be sufficiently reliable.
.P
Using the
.B MODULE_INIT_IGNORE_VERMAGIC
flag indicates that the "vermagic" string is to be ignored, and the
.B MODULE_INIT_IGNORE_MODVERSIONS
flag indicates that the symbol version hashes are to be ignored.
If the kernel is built to permit forced loading (i.e., configured with
.BR CONFIG_MODULE_FORCE_LOAD ),
then loading continues, otherwise it fails with the error
.B ENOEXEC
as expected for malformed modules.
.P
If the kernel was build with
.BR CONFIG_MODULE_DECOMPRESS ,
the in-kernel decompression feature can be used.
User-space code can check if the kernel supports decompression
by reading the
.I /sys/module/compression
attribute.
If the kernel supports decompression,
the compressed file can directly be passed to
.BR finit_module ()
using the
.B MODULE_INIT_COMPRESSED_FILE
flag.
The in-kernel module decompressor supports the following compression algorithms:
.P
.RS 4
.PD 0
.IP \[bu] 3
.I gzip
(since Linux 5.17)
.IP \[bu]
.I xz
(since Linux 5.17)
.IP \[bu]
.I zstd
.\" commit 169a58ad824d896b9e291a27193342616e651b82
(since Linux 6.2)
.PD
.RE
.P
The kernel only implements a single decompression method.
This is selected during module generation accordingly to the compression method
chosen in the kernel configuration.
.SH RETURN VALUE
On success, these system calls return 0.
On error, \-1 is returned and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.BR EBADMSG " (since Linux 3.7)"
Module signature is misformatted.
.TP
.B EBUSY
Timeout while trying to resolve a symbol reference by this module.
.TP
.B EFAULT
An address argument referred to a location that
is outside the process's accessible address space.
.TP
.BR ENOKEY " (since Linux 3.7)"
.\" commit 48ba2462ace6072741fd8d0058207d630ce93bf1
.\" commit 1d0059f3a468825b5fc5405c636a2f6e02707ffa
.\" commit 106a4ee258d14818467829bf0e12aeae14c16cd7
Module signature is invalid or
the kernel does not have a key for this module.
This error is returned only if the kernel was configured with
.BR CONFIG_MODULE_SIG_FORCE ;
if the kernel was not configured with this option,
then an invalid or unsigned module simply taints the kernel.
.TP
.B ENOMEM
Out of memory.
.TP
.B EPERM
The caller was not privileged
(did not have the
.B CAP_SYS_MODULE
capability),
or module loading is disabled
(see
.I /proc/sys/kernel/modules_disabled
in
.BR proc (5)).
.P
The following errors may additionally occur for
.BR init_module ():
.TP
.B EEXIST
A module with this name is already loaded.
.TP
.B EINVAL
.I param_values
is invalid, or some part of the ELF image in
.I module_image
contains inconsistencies.
.\" .TP
.\" .BR EINVAL " (Linux 2.4 and earlier)"
.\" Some
.\" .I image
.\" slot is filled in incorrectly,
.\" .I image\->name
.\" does not correspond to the original module name, some
.\" .I image\->deps
.\" entry does not correspond to a loaded module,
.\" or some other similar inconsistency.
.TP
.B ENOEXEC
The binary image supplied in
.I module_image
is not an ELF image,
or is an ELF image that is invalid or for a different architecture.
.P
The following errors may additionally occur for
.BR finit_module ():
.TP
.B EBADF
The file referred to by
.I fd
is not opened for reading.
.TP
.B EFBIG
The file referred to by
.I fd
is too large.
.TP
.B EINVAL
.I flags
is invalid.
.TP
.B EINVAL
The decompressor sanity checks failed,
while loading a compressed module with flag
.B MODULE_INIT_COMPRESSED_FILE
set.
.TP
.B ENOEXEC
.I fd
does not refer to an open file.
.TP
.BR EOPNOTSUPP " (since Linux 5.17)"
The flag
.B MODULE_INIT_COMPRESSED_FILE
is set to load a compressed module,
and the kernel was built without
.BR CONFIG_MODULE_DECOMPRESS .
.TP
.BR ETXTBSY " (since Linux 4.7)"
.\" commit 39d637af5aa7577f655c58b9e55587566c63a0af
The file referred to by
.I fd
is opened for read-write.
.P
In addition to the above errors, if the module's
.I init
function is executed and returns an error, then
.BR init_module ()
or
.BR finit_module ()
fails and
.I errno
is set to the value returned by the
.I init
function.
.SH STANDARDS
Linux.
.SH HISTORY
.TP
.BR finit_module ()
Linux 3.8.
.P
The
.BR init_module ()
system call is not supported by glibc.
No declaration is provided in glibc headers, but, through a quirk of history,
glibc versions before glibc 2.23 did export an ABI for this system call.
Therefore, in order to employ this system call,
it is (before glibc 2.23) sufficient to
manually declare the interface in your code;
alternatively, you can invoke the system call using
.BR syscall (2).
.SS Linux 2.4 and earlier
In Linux 2.4 and earlier, the
.BR init_module ()
system call was rather different:
.P
.B "    #include <linux/module.h>"
.P
.BI "    int init_module(const char *" name ", struct module *" image );
.P
(User-space applications can detect which version of
.BR init_module ()
is available by calling
.BR query_module ();
the latter call fails with the error
.B ENOSYS
on Linux 2.6 and later.)
.P
The older version of the system call
loads the relocated module image pointed to by
.I image
into kernel space and runs the module's
.I init
function.
The caller is responsible for providing the relocated image (since
Linux 2.6, the
.BR init_module ()
system call does the relocation).
.P
The module image begins with a module structure and is followed by
code and data as appropriate.
Since Linux 2.2, the module structure is defined as follows:
.P
.in +4n
.EX
struct module {
    unsigned long         size_of_struct;
    struct module        *next;
    const char           *name;
    unsigned long         size;
    long                  usecount;
    unsigned long         flags;
    unsigned int          nsyms;
    unsigned int          ndeps;
    struct module_symbol *syms;
    struct module_ref    *deps;
    struct module_ref    *refs;
    int                 (*init)(void);
    void                (*cleanup)(void);
    const struct exception_table_entry *ex_table_start;
    const struct exception_table_entry *ex_table_end;
#ifdef __alpha__
    unsigned long gp;
#endif
};
.EE
.in
.P
All of the pointer fields, with the exception of
.I next
and
.IR refs ,
are expected to point within the module body and be
initialized as appropriate for kernel space, that is, relocated with
the rest of the module.
.SH NOTES
Information about currently loaded modules can be found in
.I /proc/modules
and in the file trees under the per-module subdirectories under
.IR /sys/module .
.P
See the Linux kernel source file
.I include/linux/module.h
for some useful background information.
.SH SEE ALSO
.BR create_module (2),
.BR delete_module (2),
.BR query_module (2),
.BR lsmod (8),
.BR modprobe (8)
